Creating and indexing lists

A list of values

A list holds several values in order, written in square brackets:

colours = ["red", "green", "blue"]
mixed = [1, "two", 3.0, True]
empty = []

Python does not mind what you put in a list, though in practice keeping one type per list saves you trouble.

You reach a single item by its index, and indexes start at zero:

colours[0]    # "red"
colours[2]    # "blue"

Negative indexes count from the end, which saves the arithmetic:

colours[-1]   # "blue" — the last one
colours[-2]   # "green"

Asking for an index that does not exist raises IndexError. len(colours) gives you the count, so the last valid index is always len(colours) - 1.

Your turn: pull the first, third and last planets out of the list.

Changing a list

Lists are mutable — you can change them after creating them:

colours[0] = "crimson"      # replace in place
colours.append("yellow")    # add to the end
colours.insert(1, "amber")  # add at a position

append is the workhorse. Note that it changes the list and returns None, so colours = colours.append("x") is a classic mistake that throws your list away.

From here on the exercises are written as functions, the shape you met in Python Basics: def name(parameter):, an indented body, and return to hand the answer back. You never call it yourself — the tests do, with lists of their own — so items is whatever they pass in, and return is how they see your result.

Your turn: write add_and_replace(items) that replaces the first item with "first", appends "last", and returns the list.